
public class LoopQueue<E> implements Queue<E> {

    private E[] data;
    private int front, tail;
    private int size;

    public LoopQueue(int capacity){
        data = (E[])new Object[capacity + 1];   // 其中会有一个空的元素
        front = tail = size = 0;
    }
    public LoopQueue(){
        this(10);
    }

    public int getCapacity(){
        return data.length - 1;
    }

    @Override
    public boolean isEmpty(){
        return front == tail;
    }

    @Override
    public int getSize(){
        return size;
    }

    @Override
    public void enqueue(E e){

        if ((tail + 1) % data.length == front) {
            resize(getCapacity() * 2);
        }

        data[tail] = e;
        tail = (tail + 1) % data.length;
        size ++;

    }

    @Override
    public E dequeue(){
        if (isEmpty())
            throw new IllegalArgumentException("Queue is empty");
        E ret = data[front];

        data[front] = null;
        front = (front +1) % data.length;
        size --;
        if (size == getCapacity() / 4 && getCapacity() / 2 != 0) {
            resize(getCapacity() / 2);
        }
        return ret;
    }

    @Override
    public E getFront(){
        if (isEmpty())
            throw new IllegalArgumentException("Queue is empty");
        return data[front];
    }

    private void resize(int newCapacity){
        E[] newData = (E[])new Object[newCapacity + 1];
        for (int i = 0; i < size; i ++){
            newData[i] = data[(front + i) % data.length];
        }
        data = newData;
        front = 0;
        tail = size;
    }

    @Override
    public String toString(){
        StringBuilder res = new StringBuilder();
        res.append(String.format("Queue:size = %d, capacity = %d\n", size, getCapacity()));
        res.append("front [ ");
        for (int i = front; i != tail; i = (i +1) % data.length) {
            res.append(data[i]);
            if ((i + 1) % data.length != tail) {
                res.append(',');
            }
        }
        res.append("] tail");
        return res.toString();
    }

    public static void main(String[] args){
        LoopQueue<Integer> queue = new LoopQueue<>();
        for (int i = 0; i < 5; i ++){
            queue.enqueue(i);
            System.out.println(queue);

            if (i % 3 == 2){
                queue.dequeue();
                System.out.println(queue);
            }
        }
        queue.enqueue(12);
        System.out.println(queue);
        System.out.println("getCapacity() = " + queue.getCapacity());

        queue.enqueue(13);
        System.out.println(queue);
        System.out.println("getCapacity() = " + queue.getCapacity());

        queue.enqueue(14);
        System.out.println(queue);
        System.out.println("getCapacity() = " + queue.getCapacity());

        queue.enqueue(15);
        System.out.println(queue);
        System.out.println("getCapacity() = " + queue.getCapacity());
    }
}
